Acoustic well logging tools have long been used for evaluating cementing conditions of casings in cased wells. Cement bonding is a term which has been used to describe the measure of the average compressive strength of cement disposed around a section of casing within a wellbore. This measure of cement compressive strength provides an indication of the cementing conditions within the wellbore, such as whether the cement has properly cured, whether there are voids or channels within the cement, or whether there is cement behind the casing at all.
In general, cement bonding measurements provide an indication of the cement placement behind the wellbore casing for determining whether there is a fluid seal of the casing. Having a fluid seal of the wellbore serves to protect the wellbore from fluids that may flow between portions of the wellbore. Also, another function of having an adequate cement bond is to provide mechanical support for the wellbore. Because the wellbore may be used as a means for conveying valves, packers, pumps, etc., the wall of the casing may become thinned. Therefore having adequate cement bonding is important for both providing structural support for the wellbore as well as for providing a necessary fluid seal.
Acoustic methods have been used in the industry for measuring the cementing conditions in a wellbore. These methods involve measuring the responses of acoustic signal reflection from the wellbore casing. By analyzing the acoustic signal responses from the wellbore casing, many cementing conditions have been determined such as cement compressive strength, the thickness of the wellbore casing, and even defects or the amount of corrosion that may exist within the inner surface of the wellbore casing itself.
Examples of methods and apparatuses for measuring acoustic signals for determining cementing conditions within a wellbore are disclosed in U.S. Pat. App. 2006/0067162, by Blankinship et al., entitled “Ultrasonic Cement Scanner” and U.S. Pat. No. 5,377,160, by Tello et al., entitled “Transmitter and Receiver to Radially Scan the Cementing Conditions in Cased Wells,” which are both incorporated herein.
Many of these methods and apparatuses for measuring acoustic signals use a combination of sector cement bond logging (CBL) transmitters with receivers and monopole CBL transmitters having corresponding monopole CBL receivers. These CBL transmitters are typically used in a wellbore to transmit acoustic signals into the wellbore fluid and to the wellbore casing, the reflection thereof received by a corresponding CBL receiver and analyzed to determine the cementing conditions of the wellbore casing.
However, each combination of sector cement bond logging (CBL) transmitters with receivers and monopole CBL transmitters with corresponding monopole CBL receivers have corresponding strengths and weaknesses. For example, where transmitting acoustic signals between monopole CBL transmitters and monopole CBL receivers may illustrate the amount cement has bonded all the way around the wellbore casing, (i.e., how well the cement is bonded around the inside of the casing as a whole), they cannot illustrate how the cement has bonded to any particular section of the pipe. In contrast, the acoustic signals measured between sector transmitters and sector receivers illustrate a picture of the individual channels (i.e., individual sectors or sections of the wellbore casing).
FIG. 1A illustrates a wellbore log 100 of a wellbore for presenting cement bonding data according to the prior art. The first column 106 depicts standard cement bond logging information for a monopole receiver displaced 3 feet from a monopole transmitter in a wellbore. The second column 107 depicts the cement bonding information for sector receivers displaced 2 feet from the sector transmitters, while the fourth column 109 depicts a standard Variable Density Log (V.D.L.) presentation for the monopole receiver displaced 5 feet from a monopole transmitter. Further, the third column 108 shows a depiction of the variable amplitudes within a circumferential placement of cement about the exterior circumference of the casing, with darker shading showing a measurement of higher compressive strength for the cement. As shown, by knowing the data obtained from sector receivers displaced 2 feet from the sector transmitters it is possible to identify the circumferential qualities of the wellbore casing at given pressures at specific sections of the casing.
Therefore, by measuring and analyzing the individual sectors of the wellbore it is possible to determine if there is good bonding everywhere except for an individual channel or sector (e.g., instead of potentially making the assumption that there exists a bad cement job throughout the outer circumference of the casing). That is, although it is possible to measure the overall poor compressive strength of the wellbore as a whole at a certain depth using monopole CBL data, with sector CBL data it is possible for example to identify whether there actually exists good cementing conditions around the majority of the wellbore casing, except that the wellbore casing is leaning against a rock formation and the cement was unable to bond to a particular sector. As a result, sector CBL acoustic signal evaluation will tell you whether there exist poor compressive strength of the cement, or whether you have good compressive strength of the cement, but a poorly bonded channel.
However, one downfall of using sector CBL transmit to sector CBL receive is that, depending on the conditions downhole, acoustic signal measurements between sector CBL transmitters and sector CBL receivers are not as stable as similar measurements between monopole CBL transmitters and monopole CBL receivers. This is because the acoustic signal measurements between the sector CBL transmitters and sector CBL receivers are very sensitive to transmitter/receiver matching, and more, to changes in pressure and temperature downhole. Because the pressure and temperatures downhole will typically vary greatly, the acoustic signal amplitudes measured at the sector CBL receivers may suffer a substantial drift in amplitude, making the resulting data unstable and unreliable.
As a result, not only does the adequacy of the cementing conditions downhole affect the amplitudes measured at the sector CBL receivers, but also temperature and pressure changes downhole. Therefore, by only analyzing the amplitude of the measured acoustic signals themselves it is impossible to determine whether amplitude variations are caused by the cementing conditions or the changes in temperature and pressure downhole.
Because of the above identified problems with acoustic signal measurements between sector CBL transmitters and sector CBL receivers, many operators have given up completely on sector transmit to sector receive, either relying entirely on using monopole CBL transmitters and monopole CBL receivers or using a combination of a single monopole transmitter and sector receivers. Although, the above attempts to avoid the pressure/temperature sensitivities of sector CBL transmitter to sector CBL receiver are possible, transmitting an acoustic signal with a single monopole transmitter and receiving the acoustic signal using sector receivers severely sacrifices the radial sensitivity attained by transmitting with sector transmitters and receiving with sector receivers (i.e., radial resolution around the pipe). Further, the spatial resolution of the sector receivers deteriorates as their distance from the transmitter increases.
It is therefore desirable to have a system and method that allows accurately measuring the cementing conditions downhole by correcting the instability and drift in amplitude of measured acoustic signals due to changes in pressure and temperature downhole, thereby preserving the radial sensitivity obtained by transmitting from sector CBL transmitters and receiving at sector CBL receivers.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.